Human intestinal absorption of chemical compounds can depend simultaneously on several key properties: dissolution, precipitation, solubility, permeability, and, if the compound is ionizable—pKa. This association is exemplified by the compound classification approaches well known to those in pharmaceutical research, e.g., the Absorption Potential, the Biopharmaceutics Classification System, and the Maximum Absorbable Dose classification.
In pharmaceutical research, looking for a new drug typically takes place in three stages: exploration, discovery, and development. In the first stage, the understanding of the disease state is accumulated, a therapeutic target is selected, and a biological screening assay is developed. The discovery stage begins with ‘hits’ finding, where a company's library of compounds is screened for the IC50 value, the concentration of the compound required to displace 50% of a reference ligand from a target receptor. In the course of a year at a large pharmaceutical company, it is not uncommon to have 100,000 to 1,000,000 library compounds tested against a particular target, which is usually a receptor site on a protein molecule. Of the molecules tested for biological activity, about 3,000 to 10,000 are typically found to be active (hits). The initial part of the discovery step is called ‘lead’ generation, where the most promising subset of the hits is selected for further testing. Of the 3,000-10,000 potent molecules, about 400 make it to this step. The selection of leads takes into account biopharmaceutic properties of the hits, such as measured aqueous solubility, octanol-water partition coefficients, plasma stability, human serum protein binding, cytochrome P450 inhibition (oxidative metabolism), liver microsome assay (general metabolism), and membrane permeability, using an in vitro cultured-cell model, such as Caco-2. These various tests filter out many molecules with unfavorable biopharmaceutic ADME properties (absorption, distribution, metabolism, and excretion). Most companies perform fast ADME screens in the hits-to-leads transition to aid in “go—no go” decisions. The selected 400 lead compounds are expected to have good in vivo pharmacokinetic (PK) behavior in animal models developed later. But many of the molecules will underperform in laboratory animals, and will be rejected. In lead optimization, the compounds are rigorously tested for in vitro ADME properties, central nervous system (CNS) penetration, selectivity against other similar targets, as well as for cytotoxicity. In the final stages of optimization, where rodent in vivo PK measurements are done, metabolic profiles are developed, and additional animal model toxicity tests are performed, about twelve promising ‘candidate’ molecules typically survive to enter pre-clinical development, where dosage form design and human PK, safety, and effectiveness testing begin. During the subsequent clinical phases, the number of clinical development molecules dwindles down to about one, a considerable and costly downsizing from the original 400 promising leads.
ADME was the single largest cause of attrition in drug development, accounting for up top 40% of the failures, based on a 1997 study. Since the early 2000s, methods and systems for estimating ADME properties have been introduced, and this has led to significant reductions in the ADME-based attrition rates, benefiting the industry by reducing costs and the consumers by helping to get better drugs to market in less time.
The present application relates particularly to four important physicochemical properties related to ADME: dissolution, precipitation, solubility, and permeability. Collectively, these are properties underlying drug absorption—the ‘A’ in ADME.
Dissolution (also herein referred to as solubilization), refers to a dynamic process involving the kinetics by which a chemical compound dissolves into a given medium (e.g., solvent). Related to this is solubility, either equilibrium solubility (of the thermodynamically most-stable form of the compound) and/or kinetic solubility (of the active polymorph or salt form of the compound). Also related to this is precipitation, which refers to a dynamic process where a solid in an active polymorphic form first dissolves but then (often, spontaneously) precipitates as a new solid in a more stable polymorphic form. Precipitation is also effected when a supersaturated solution of a compound starts to precipitate a solid form of the compound, usually in a particular polymorphic form. Precipitation may also be triggered following an initial process of dissolution, when a dissolved compound is exposed to a changed solution (e.g., by alteration of pH of the medium, or ionic strength, or buffer constituents, or other additives), such that the compound becomes supersaturated and begins to precipitate as a solid. In this view, precipitation and dissolution are inversely related processes, which can be studied by similar analytical methods and systems. Viewed from the perspective of the resulting solution, dissolution can be characterized by the time-rate-of-change in concentration of the compound in the solution over a dissolution period. In contrast, solubility most often refers to an equilibrium condition (e.g., a time-invariant thermodynamic value) and particularly refers to how much of a sample compound will dissolve into a given medium under conditions in which thermodynamic equilibrium is achieved. Compounds that have a high solubility will generally demonstrate faster dissolution than compounds of lower solubility. However, dissolution characteristics may not directly correlatable to solubility, and valuable information about compounds can be obtained by examining dissolution profiles, in addition to overall solubility data. Dissolution testing of compounds is typically practiced by dissolving at least a portion of a compound in a solvent to form a solution that has a varying concentration of the compound over a dissolution period. The concentrations of the compound dissolved in solution are measured at various times during the dissolution period. This information, taken collectively, represents a time-dependent dissolution profile. If allowed enough compound and enough time to dissolve to reach saturation (i.e., indicated by the presence of excess solid suspended in the solution at equilibrium), one could also measure solubility. In addition, the kinetic solubility of the active polymorph or salt form of the compound may be estimated from the initial dissolution characteristics. However, a dissolution profile can be determined without necessarily determining solubility. Dissolution and precipitation testing is known in many fields, but is of particular significance with respect to drug candidates.
In addition, the present application relates to the measurement of membrane permeability of compounds (also herein referred to as permeability), referring to a dynamic process involving the kinetics by which a compound is transported across a membrane barrier in a permeation cell comprising two solutions in contact with a semi-permeable membrane that lies between and separates the two solutions.
Dissolution testing is required as quality control (QC) for marketed pharmaceutical dosage forms in which gastrointestinal absorption of the drug is necessary for the desired therapeutic effect. The U.S. Pharmacopoeia (USP), through its system of monographs, is one well-known standard source of methods for dissolution and drug release testing. USP dissolution methods employ large volumes of dissolution media (usually 500 or 900 mL), continuously agitated and maintained at 37° C. At fixed time intervals, a small aliquot of sample is taken from each solution vessel, by a multi channeled pumping system, then filtered, and transported to a sample vial for subsequent spectrophotometric or high pressure liquid chromatography (HPLC) analysis. Plotting the percentage of dissolution of a pharmaceutical dosage form as a function of time results in a dissolution profile.
Apart from the above QC applications, dissolution testing is also used during drug formulation development (FD). Precipitation studies often accompany those of dissolution, especially when polymorphic transformations are suspected or are known to occur.
The need for dissolution testing may even be emerging further upstream, in drug discovery (DD). Although high-throughput methods in DD have increased the speed of identification of biologically active compounds, bottlenecks have emerged, impeding timely introduction of new drugs to market. One such bottleneck is the identification of drug candidates that are soluble and/or that have an appropriate rate of solubilization in an aqueous solution such as water or buffered water. Low solubility and/or solubilization of a drug candidate can be problematic because it can make the drug difficult to deliver effectively in a biological system, thus reducing bioavailability. It has been estimated that as many as 12-40% of drug candidates fail in animal toxicology and/or clinical trials because they are poorly soluble and/or have poor solubilization.
Approaches to solve solubility and/or solubilization problems include identification of salt forms or related structures (e.g., polymorphs) of the drug candidate that may show equivalent activity but improved solubility and/or solubilization. However, such methods of identification (involving, e.g., design, synthesis, and characterization of salt/polymorphic forms of a drug candidate) are generally time consuming, tedious and are by themselves bottlenecks in getting a new drug to market.